The goal of this project is to use quantitative neuropathologic methods to define mild cognitive impairment (MCI) in the elderly and its relationship to Alzheimer's disease (AD). The focus is on a select group of markers for different stages of a pathogenetic sequence that we hypothesize begins with deposition of amino-terminally modified or truncated AB followed by deposition of full-length Ap that leads to neuroinflammation and subsequent cellular stress that manifests in aberrant activation of the cell cycle and cytoskeletal pathology in neurons and eventual synaptic loss. We hypothesize that biochemical alterations are widespread in the brain and not only in areas with obvious structural changes. We also hypothesize that there may be regional differences with respect to significant synaptic loss. Previously, we analyzed frontal lobe, but we will analyze cortical areas theoretically more (temporal) or less (occipital) vulnerable than the frontal lobe. The specific aims are: 1) To analyze amyloid peptide heterogeneity using assays for amino-terminal modifications or truncation as well as full length A-beta, which we hypothesize deposits first as A-beta-1-42 and later as A-beta-l-40; 2) To analyze markers of neuroinflammation and astrocytosis, which we hypothesize is due to the changing nature of amyloid in the brain, by measuring microglial activation (HLA-DR and IL1a), astrocytic gliosis (GFAP) and proinflammatory cytokines (IL-l-beta, TNF-alpha, IL-6 and IL-1ra.); 3) To analyze cytoskeletal changes that we hypothesize may be the product of aberrant activation of the cell cycle due to cellular stress due to amyloid or neuroinflammation by extending tau assays to include assays for cyclin-dependent kinase-2 and downstream effects of cell cycle activation, specifically, markers of apoptosis (activated caspase-3 and caspase-3-cleaved fodrin); 4) To analyze markers indicative of neuronal structure and number, including synaptic markers (synaptophysin, SNAP25 and synuclein), NeuN and alpha-internexin; and finally, 5) To complete studies on age-related changes in white matter that may be structural correlates of cognitive and motor slowing in normal aging with immunoassays and immunocytochemistry for ubiquitin and myelin basic protein on additional normal elderly cases. The data will be analyzed across all diagnostic categories and clinical parameters to validate the proposed pathogenetic cascade. A profile of individual MCI cases will also be analyzed and compared to the average values of normals and AD to address the issue of heterogeneity of MCI and specifically to define an MCI phenotype that most closely resembles AD.